1. Field
The present invention relates to robotic systems, and in particular to a robot for servicing the heat exchanger tubes in the steam generators of a nuclear power plant.
2. Description of the Related Art
In a pressurized water nuclear-powered electric generating system, the heat generated by the nuclear reaction is absorbed by a primary coolant that circulates through the reactor core and is used to generate steam in a steam generator. The steam generator typically is an upright cylindrical pressure vessel with hemispherical end sections. A traverse plate called a tubesheet, located at the lower end of the cylindrical section, divides the steam generator into a primary side, which is the lower hemispherical section below the tubesheet and the inside of U-shaped tubes which extend into the cylindrical section from the underside of the tubesheet, and a secondary side, which is located above the tubesheet around the outside of the tubes. A vertical wall bisects the primary side into an inlet section and an outlet section. The tubesheet is a thick carbon steel plate with an array of holes, into which the ends of U-shaped tubes are inserted. The number of tubes in a steam generator tubesheet range from about 4,000 to 15,000. One end of each U-shaped tube is inserted into a hole within the tubesheet that communicates with the inlet section of the primary side, and the other end is inserted into a hole within the tubesheet that communicates with the outlet section. The primary coolant is introduced under pressure into the inlet section of the primary side, circulates through the U-shaped tubes, and exits through the outlet section. Water introduced into the secondary side of the steam generator circulates around the U-shaped tubes and is transformed into steam by heat transferred from the primary coolant. The steam is then used to perform useful mechanical work, such as to operate a turbine-driven electric generator.
Degradation of the tubes may occur during operation of the steam generator. This degradation is undesirable, among other reasons, because the primary coolant is radioactive, so any leakage of the coolant into the secondary side of the generator contaminates the steam that contacts the turbine. It is generally not practical to replace degraded tubing. Instead, the steam generator is periodically inspected, and the affected tubes are either repaired or plugged at both ends. In view of the thousands of tubes in the steam generator, plugging a few tubes does not appreciably impact the efficiency of heat transfer.
Because of the radiation hazard present in steam generators used in a nuclear-powered facility, the heat exchanger tubes of such steam generators must be, for the most part, remotely serviced to avoid exposing maintenance personnel to potentially harmful radiation. Consequently, a number of robotic systems have been developed for remotely performing repair and maintenance operations on these heat exchanger tubes. These robotic systems typically include some sort of robotic delivery arm in combination with any one of a number of specialized tools designed to be carried by the robotic arm.
Servicing of nuclear steam generators has changed dramatically in the last twenty years. In the past, much of the older steam generator tubing became degraded and required significant plugging, sleeving, or total steam generator replacement. Power plant service outages were of long duration and the repairs to steam generators required sophisticated robotic manipulators and tooling with the capability to perform varied inspections and repairs while avoiding plugged tubes.
Presently, most utilities in the industry have either replaced their steam generators or have generators with few plugged tubes that require minimal repair. Most of the work for the steam generator tubing involves eddy current inspection with few or no plugs required. Outage time for plant maintenance and refueling, which dictates the length of the outage, has been reduced, significantly decreasing the time allotted for steam generator inspection and service. There is a clear need to improve upon existing robotic systems for servicing steam generators to meet modern maintenance timetables. Current maintenance robots are generally heavy (over 100 lbs.) and highly sophisticated, which requires more time and personnel radiation exposure for transportation, set up, and installation. Current robots are also too large to install multiple units on a given section of most steam generator channel heads.
Additionally, steam generator robotics generally have three or more degrees of freedom along with multiple motors with position feedback. The control systems, along with the complexity of the robots, are generally expensive to purchase and maintain. Furthermore, most robots, when installed in the steam generator, are anchored in place and must be moved to gain access to all the tubes during inspection.
Most tubesheet walker robots, such as the present invention, use the holes in the tubesheet to anchor the robot with a number of grippers, such as camlocks. These robots use two sets of camlocks, alternating gripping with one set and moving the other set to a new location, thereby propelling the robot across the tubesheet. A typical robot of this design is described in U.S. Pat. No. 7,314,343, entitled “Miniature Manipulator for Servicing the Interior of Nuclear Steam Generator Tubes.” The referenced design uses an “X” layout consisting of a base member that holds process specific tooling, a central rotary joint, and a linear sliding joint. In addition, there is a small amount of vertical movement in the center joint to move the robot up toward the tubesheet when necessary. The problem with the referenced design is that the mechanism is relatively complex, with many intricate parts. Also, movement can only be effected in one direction at a time, so the robot must execute multiple actions to move in any direction other than its current orientation.
As a result of the current state of the art, a simple, small, light-weight robot is desired that can facilitate tube inspection for a heat exchanger.